Ok, I've managed to get the tmpfs error to go away by compiling some extra stuff into the kernel - I'll post the fix as soon as I get past this next problem...

System boots into linux and bitches about a few missing things (nothing serious - hostname missing and sonthing about a 'logger' service missing for iptables)
During the kernel hardware detection phase, it spots the Sun Type5 keyboard I have connected and all seems well.
However, when I get to the login prompt, anything I type comes out as garbage. It's not a case of using a US layout with a UK keyboard (which I had to deal with during the install) but something else.
Hitting STOP-A gives me the PROM prompt and I can type just fine in that.

Obviously this limits what I can do to the box directly - I'll have to chroot from the ramdisk/iso to fix this.